The concentrations of melatonin in 112 preovulatory follicular fluid (FF) samples obtained from 60 women undergoing in vitro fertilization and 27 patients at laparotomy during a spontaneous cycle were measured by RIA and compared with those in peripheral serum. The circadian and seasonal variations in FF melatonin were also analyzed. The FF melatonin concentrations in stimulated (mean +/- SEM, 61.9 +/- 6.4 pmol/L) and spontaneous cycles (98.1 +/- 8.9 pmol/L) were significantly higher (P less than 0.005) than those in peripheral serum (25.4 +/- 1.2 and 38.6 +/- 1.8 pmol/L, respectively), and in the stimulated cycles there was a positive correlation between them. The FF melatonin concentration in the morning (58.9 +/- 3.8 pmol/L) was significantly higher (P less than 0.005) than that in the daytime (23.2 +/- 0.8 pmol/L), but the morning concentrations did not differ between the light and the dark seasons of the year, whereas the daytime values were higher (P less than 0.005) during the dark season (27.1 +/- 2.1 pmol/L) than during the light season (21.1 +/- 2.1 pmol/L). The FF melatonin concentration did not correlate with follicular volume, and FF and serum melatonin concentrations showed no significant correlation with the serum concentrations of estradiol, progesterone, testosterone, or PRL. There were also no differences between FF melatonin concentrations in aspirates with or without an ovum. In summary, significant circadian and circannual variations in high FF melatonin concentrations were found, which suggest that melatonin could potentially interfere with the regulation of reproduction in humans at the follicular level.